1. Field of the Invention
This invention relates to correcting the drift associated with the determination of position and flux in variable reluctance devices such as solenoids. This drift is present in systems that integrate a measured parameter of the device in order to determine flux and position.
2. Description of the Prior Art
The prior art related to the present invention includes many techniques to determine the position of a solenoid. Some systems add a position sensor, which increases the cost and complexity. Others have attempted to infer position from signals that either exist on the system or are cheaper to generate than including a position sensor. Jayawant (U.S. Pat. NO. 5,467,244) describes a measurement system that balances the position of a solenoid. Stupak (U.S. Pat. No. 4,659,969) uses a Hall sensor to measure flux. Gingrich (U.S. Pat. No. 4,368,501) shows how to generate flux (.PHI.) from a second winding. The desirability of flux information comes from the approximation equation relating flux (.PHI.), measured current (I) through the winding, and position: EQU x'=I/.PHI. Equation 1
where x' is only an approximation of true position x. In many systems the approximation is either good enough, or it is not too difficult to transform x' into x.
Flux (.PHI.) can be obtained from the applied inductive voltage VL, as: EQU VL =Vapplied-I * R Equation 2
where I is the measured current, R is the resistance of the coil in the solenoid, and Vapplied is the voltage applied across the coil. VL and flux are related with the equation: ##EQU1##
where n is the number of turns on the solenoid.
Integrating equation 3 gives: ##EQU2##
where k2 is a constant having to do with the initialization of the integrator.
Therefore the known applied voltage can be integrated, combined with the measured current, and used to obtain x' for a reasonably behaved device. But a problem in all of the systems that measure flux by doing an integration, such as Gingrich's, is that over time the integral will drift. Since the value of flux drifts from its correct value, the determination of x' will also drift (equation 1). The system will be accurate when the flux integrator can be initialized to a known state, such as zero when an unpowered system is first energized. As time passes, the drift will cause the integrator to deviate more and more from the correct value. Knowing the absolute position from some other information can also be used to initialize the integrator, since equation 1 can be solved for flux (current I and position x' being known). Certain systems, such as engine valves, quickly move from one known location to another and so are not a problem. But many valves and solenoids must hold a driven position for long periods of time, and cannot rely upon the chance that they will move to a known boundary condition and allow the integrator to be corrected within the short period of time that drift remains a small error.
The derived measurement of flux is not the only way to control position. Others have made systems that attempt to control the position of a solenoid by holding a constant current in the coil of the solenoid without employing feedback. Still others have attempted to measure the inductance of the device, since inductance can usually be converted into position for this type of device. One such method is to apply a known high frequency drive and use the resultant high frequency ripple in a measured signal such as current to calculate inductance. The constant current drive method has errors relating to the lack of any feedback, and the inductance measurement technique has been hampered with noise and measurement accuracy problems.